Xylose metabolism in Pachysolen tannophilus: purification and properties of xylose reductase

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Abstract
Xylose reductase (xylitol: NADP oxidoreductase, EC 1.1.1.10) was purified from D-xylose grown cells of the yeast P. tannophilus by application of DEAE-cellulose ion exchange chromatography, 2'',5''-ADP-Sepharose affinity chromatography, Biogel P200 gel filtration and dextran blue Sepharose chromatography to .apprx. 95% homogeneity. It consists of a single polypeptide chain with a relative MW of 35,000-40,000 and an isoelectric point of pH 4.9. The enzyme has a broad substrate specificity similar to that of aldose (or aldehyde) reductases from mammalian tissues. It exhibits Michaelis-Menten type kinetics (Km D-xylose, 162 mM; KmD-xyhtol, 212 mM; Km NADPH, 0.059 mM; Km NADP, 0.071 mM). The enzyme is specific for NADPH; activity with NADH is below 0.5% of Vmax observed with NADPH. The reduction of xylose is inhibited by NADP, the anabolic reduction charge (NADPH/NADP + NADPH) and also in a complex manner by ATP. At physiological pH values the equilibrium is Keq = 10-10. The importance of these findings for the physiology of xylose fermentation by this yeast is discussed.

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